[xubuntu] How can switch DE between xubuntu and unity?

I installed xubuntu and then I want to switch back to the default unity DE, ideally I intend to swith beteen those two kind of DE smoothly. How can I do that?

The ubuntu I installed is 16.04 LTS. By now, everytime, when I boot up, it will show a login screen from xubuntu.

Thanks in advance